Electrical transmission and distribution equipment is subject to voltages within a fairly narrow range under normal operating conditions. However, system disturbances, such as lightning strikes and switching surges, may produce momentary or extended voltage levels that greatly exceed the levels experienced by the equipment during normal operating conditions. These voltage variations often are referred to as over-voltage conditions.
If not protected from over-voltage conditions, critical and expensive equipment, such as transformers, switching devices, computer equipment, and electrical machinery, may be damaged or destroyed by over-voltage conditions and associated current surges. Accordingly, it is routine practice for system designers to use surge arresters to protect system components from dangerous over-voltage conditions.
A surge arrester is a protective device that is commonly connected in parallel with a comparatively expensive piece of electrical equipment so as to shunt or divert over-voltage-induced current surges safely around the equipment, thereby protecting the equipment and its internal circuitry from damage. When exposed to an over-voltage condition, the surge arrester operates in a low impedance mode that provides a current path to electrical ground having a relatively low impedance. The surge arrester otherwise operates in a high impedance mode that provides a current path to ground having a relatively high impedance. The impedance of the current path is substantially lower than the impedance of the equipment being protected by the surge arrester when the surge arrester is operating in the low-impedance mode, and is otherwise substantially higher than the impedance of the protected equipment.
When the over-voltage condition has passed, the surge arrester returns to operation in the high impedance mode. This high impedance mode prevents normal current at the system frequency from flowing through the surge arrester to ground.
Conventional surge arresters typically include an elongated outer enclosure or housing made of an electrically insulating material, a pair of electrical terminals at opposite ends of the enclosure for connecting the arrester between a line-potential conductor and electrical ground, and an array of other electrical components that form a series electrical path between the terminals. These components typically include a stack of voltage-dependent, nonlinear resistive elements, referred to as varistors. A varistor is characterized by having a relatively high impedance when exposed to a normal system frequency voltage, and a much lower resistance when exposed to a larger voltage, such as is associated with over-voltage conditions. In addition to varistors, a surge arrester also may include one or more spark gap assemblies electrically connected in series or parallel with one or more of the varistors. Some arresters also include electrically conductive spacer elements coaxially aligned with the varistors and gap assemblies.
For proper arrester operation, contact must be maintained between the components of the stack. To accomplish this, it is known to apply an axial load to the elements of the stack. Good axial contact is important to ensure a relatively low contact resistance between the adjacent faces of the elements, to ensure a relatively uniform current distribution through the elements, and to provide good heat transfer between the elements and the end terminals.
One way to apply this load is to employ springs within the housing to assure the stacked elements engage with one another. Another way to apply the load is to wrap the stack of arrester elements with glass fibers so as to axially-compress the elements within the stack.
Another way to assure adequate contact between components of the stack is to bond them to one another by various techniques. Components of the stack may be bonded together using a bonding technique that includes applying a preform or coil of solder between components to be bonded. In some implementations, the face of a component is attached directly to another surface by, for example, soldering or brazing directly to the surface.
The use of high temperature solders or brazing may require more aggressive fluxes when heated in air. Also, a secondary heat treatment of the varistors may be required to restore desired properties. Problems associated with high temperature solders or brazing in air may be avoided by heating in a reducing atmosphere. However, the reducing atmosphere may have an unrecoverable adverse effect on varistor properties.
When using solder, it is desirable to use low temperature solders, so as to avoid heating the MOV disks to temperatures that can damage the disks. This also tends to allow the use of less aggressive fluxes, which reduces the potential attack on the bond between components.
A potential problem associated with using low temperature solders is that, in some cases, the solder temperature (for example, 221° Celsius) can approach the operating temperature of the device (for example, 200° Celsius), which can lead to partial melting of the solder and potential device failure under extreme operating conditions. This problem may be avoided by selecting a solder having a solder temperature that differs sufficiently from the operating temperature, while not being too high.
Other techniques for attaching electrical components include the use of an organic adhesive, such as a metal-filled epoxy; an inorganic adhesive; or brazes. Each of the above techniques can be performed with or without metallized faces being deposited on surfaces of the components.
Bonding between the faces or surfaces of adjacent MOVs has typically been achieved using aggressive fluxes between the base metal applied to the MOV and a preform or a coil of solder. The fluxes are used in this case to prevent oxidation during the heating processes and also to clean or remove any dust or other contaminants on the surface to be bonded. However, the application of flux to a surface results in a porosity of the surface, due to the formation of voids or discontinuities in the surface. Furthermore, with the use of flux, a flux residue is often left under the solder. Both the increased porosity and the flux residue can potentially weaken the bond between the surfaces of elements in the stack.